In stratified charge engines, fuel is injected into a combustion chamber just before the fuel is ignited by a spark plug. Typically, a stratified charge engine has an injector positioned in the combustion chamber to inject the fuel proximate the spark plug. This allows for higher compression ratios without pre-combustion or “knock” and leaner air/fuel mixtures than in conventional internal combustion engines.
Since fuel is not present in the combustion chamber until the point of combustion beginning, there is reduced risk of pre-combustion or engine knock. The engine may also run on a much leaner overall air/fuel ratio because the stratified charge provides a richer air/fuel mixture proximate the spark plug and a leaner air/fuel mixture elsewhere in the chamber. Accordingly, the stratified charge is non-homogenous, as opposed to air/fuel mixtures of typical spark-ignition engines. A higher mechanical compression ratio may also be used in stratified charge engines for better thermodynamic efficiency.
In some cases, the stratified charge engines experience improved efficiency due to the leaner air/fuel mixtures and due to reduced pumping losses. Pumping losses are caused by work required to move air into and out of the cylinders, where the losses increase with engine speed. The pumping losses are increased significantly with a reducing load when throttling. Stratified charge engines may only exhibit improved efficiency in a limited load range as the engine reaches air intake limits at lower load levels due to an increased amount of air and exhaust gas required in the combustion process.